1. Field of the Invention
This invention relates to a semiconductor power converter apparatus in which a plurality of semiconductor elements are connected in parallel.
2. Description of Related Art
In a semiconductor power converter apparatus using power semiconductor elements, such as represented by IGBTs (Insulated Gate Bipolar Transistors), the IGBTs are caused to perform switching operations and the proportions of the current passing therethrough are controlled, to adjust the electric power flow. Passage of an overcurrent through such a semiconductor power converter apparatus elevates the temperature of the IGBTs, which may result in destruction of the semiconductor power converter apparatus. A variety of protection circuits for preventing the IGBTs from the destruction have been proposed which detects the temperature of the IGBTs with a temperature sensing unit to stop the operation of the IGBTs or to limit the passing current through the IGBTs before the IGBTs result in destruction.
For example, JP-B-3194353 describes a method for protecting IGBTs against overheating in the case where two chips of IGBTs are provided in parallel, each of the IGBTs having a diode for detecting temperatures thereon. The diodes are connected in parallel, and the forward voltage of the diodes is monitored while passing a constant current through the diodes from a single current source. Here, because the forward voltage of diodes shows such a profile that it decreases linearly with respect to the temperature, the temperature detection device is able to find that the detected temperature has reached a desired temperature through monitoring the forward voltage.
In addition, according to JP-A-2004-80865, a plurality of temperature sensing diodes connected in series are categorized into at least two separate groups, and a current is supplied from a constant current circuit to the temperature sensing diodes connected in series. A difference in the series voltage drop between the respective groups is obtained with a differential circuit. If the difference voltage exceeds a predetermined voltage, it is determined that one of the IGBTs is in an abnormal condition, so a signal indicating that is issued. Thereby, even when a temperature rise occurs in one among the plurality of the semiconductor elements, the temperature of the semiconductor elements can be detected accurately.
In the conventional example disclosed in JP-B-3194353, a problem has been that, since a plurality of temperature sensing diodes are connected in parallel, a variation in the characteristics of the temperature sensing diodes causes an imbalance in the currents flowing through the temperature sensing diodes, degrading the accuracy of the IGBTs' temperatures detected. Another problem has been as follows. In the case where all the IGBTs operate properly, an accurate temperature can be detected to some extent. However, in the case where any one of the IGBTs undergoes an open destruction, the temperature of the IGBT that has undergone the open destruction becomes extremely lower than those of the other IGBTs and accordingly, the detected value of the temperature of the IGBTs becomes lower than the actual temperature. This may lead to destruction of the other IGBTs, like a chain reaction. Moreover, although only one current source is necessary, all the temperature sensing diodes require passage of current therethrough. This complicates the wiring structure, leading to the problems of cost increases and degradation in noise resistance performance.
In addition, the conventional example disclosed in JP-A-2004-80865 performs abnormality determination for IGBTs by making use of the fact that a temperature difference occurs between a normal IGBT and an abnormal IGBT and a forward voltage difference arises between the temperature sensing diodes in each group. Nevertheless, since the temperature sensing diodes in each group are connected in series, a difference arises between the forward voltages of the temperature sensing diodes in each group even when they are at the same temperature, because of variations in the characteristics of the temperature sensing diodes. Therefore, the threshold voltage for determining an abnormality in the IGBTs cannot be set to a very low voltage. For this reason, it is necessary to cause a temperature difference between a normal IGBT and an abnormal IGBT to perform the abnormality determination for IGBTs, and this temperature difference may cause the IGBTs to undergo thermal destruction. Moreover, this conventional example requires two current sources, and furthermore, all the temperature sensing diodes require passage of current therethrough. This complicates the wiring structure, leading to the problems of cost increases and degradation in noise resistance performance.